An inertial measurement unit (IMU) provides both inertial measurements (using gyroscopes) and acceleration measurements (using accelerometers). Gyroscopes and accelerometers, however, are both sensors inherently corrupted by measurement errors. Measurement error sources include a sensor's post-processing electronic circuitry, variation in supplied power, thermally induced variations, etc. Sensor calibration is used to remove measurement errors by estimating a sensor's error and providing adjustment corrections to the IMU in order to improve its performance (e.g., accuracy).
For IMUs with three orthogonal sense axes, conventional IMU calibration methods require successive repositioning of the IMU sense axes along the three orthogonal dimensions to appropriately exercise each sense axis. For small stand-alone IMU packages this is done with simple reconfigurable fixtures and precise rate tables. In the case of angular rates, the gyroscope output is compared against an output of a rate table, where the gyroscope's sense axis is aligned with the table's spin axis. Conventional methods for calibrating an accelerometer output are based on comparing the accelerometer output to the expected gravitational field. Thus, in order to accurately calibrate the accelerometers, a precise knowledge of the accelerometer sense axis orientation with respect to the gravitation field is required. To characterize thermally induced error variations, the rate table along with the IMU mount fixture is placed inside of a thermal chamber and the temperature is varied while the calibration of the IMU is repeated.
For large systems with integrated IMUs, such as gimbaled antennas, a successful calibration requires significantly more complicated and rugged rate tables and fixtures capable of supporting the increased weight and awkward shapes of the large systems. If the thermal chamber cannot support the weight, size, and/or the awkward shape of the integrated system, IMU calibration with respect to temperature is bypassed at the price of decreased performance.